1. Field of the Invention
The present invention relates to apparatus for growing single crystals by pulling them from a melt frozen on a seed crystal as well as to a method of withdrawing the grown crystals from each apparatus.
This invention proves most advantageous in growing large single crystals, for example crystals of above 200 mm in diameter, over 300 mm in length, and weighing tens of kilograms and more. In particular, the invention can be used in growing large optical and scintillation crystals of haloid fluorine compounds (sodium chloride, potassium chloride, potassium bromide, sodium bromide, lithium fluoride, cesium iodide, sodium iodide activated by tallium, cesium iodide activated by thallium or sodium) as well as semiconductor single crystals (germanium, silicon and others).
2. Description of the Prior Art
One of the most widely used methods of producing single crystals is by pulling them from a melt frozen on a seed crystal, which is usually monocrystalline. The method is based on the techniques of Czochralski and Kyropoulos (see J. Czochralski, Eine neues Verfahren zur Messung der Kristallisationgeschwindigkeit der Metalle.--Z. Phys. Chem., 1918, 92, 219-224 and also S. Kyropoulos, Ein Verfahren zur Herstellung gro.beta.er Kristalle.--Z. anorg. u. allgem. Chem., 1926, 154, 308-313).
According to the above technique, starting materials are heated in a crucible to produce a melt and then a monocrystalline seed held on a rotating vertical rod is lowered to touch the melt. After a small portion of the seed has been molten it is slowly raised and the temperature of the melt is lowered so that the melt begins to freeze onto the seed. As the crystal grows the temperature of the melt is controlled so as to prevent further growth when the crystal has acquired predetermined dimensions. When the growing crystal is of a predetermined length it is rapidly pulled away from the melt by lifting the pulling rod. The grown crystal is then cooled to be used for its designated purpose.
The grown crystal may actually be cooled to room temperature above the crucible holding the solidified portion of the remaining melt, i.e. it can be done directly in the furnace growing chamber. However, since the temperature distribution throughout the furnace chamber is uneven and with the end of rapid withdrawal of the grown crystal, to make the chamber available for the next growing cycle, cooling is usually carried out in a separate furnace (an annealing furnace) wherein the crystal temperature is equalized througout its volume and subsequently lowered to the room temperature.
As the grown crystal is transferred from the growing chamber to the annealing chamber it may develop cracks and/or break away from the seed as a result of the severe thermal conditions due to high temperature gradients (thermal shock). Thermal shock is an important problem which is especially felt in growing large crystals.
Known in the art is an apparatus for growing single crystals using the Kyropoulos technique, which apparatus comprises a heated crucible to hold the melt and to provide a working volume, a lid for closing the crucible composed of two semicircular members, a pulling rod provided with a seed holder vertically extending to the crucible through the lid, and a drive for moving the pulling rod in both rotational and translational modes (see K.--T. Wilke, Vyrashchivanie Kristallov, Leningrad, Nedra Publishers, 1977, p. 330-331, FIGS. 3.2-5). The grown single crystal is withdrawn from the above apparatus as follows. In the beginning the crystal is pulled away from the melt and the lid is opened by separating its semicircular members and the pulling rod with the crystal is rapidly lifted above the lid. Following this is a horizontal transfer of the pulling rod away from the crucible whereupon the rod is lowered (e.g. into an annealing furnace) and the crystal is let loose of the seed holder.
It is however to be understood that in the process of opening the lid, withdrawing the crystal from the crucible and transferring the crystal to the annealing furnace both the seed and the crystal grown are sensitive to the action of cold air whereby thermal shock is produced that may be the cause of cracks and the crystal breakdown, as has already been stated above.
Also, starts and stops during the horizontal transfer of the crystal suspended from the seed may result in breakdown of the crystal due to inertia, especially a crystal having large dimensions and weight.
Moreover, it is to be appreciated that withdrawing the crystal through the lid requires an apparatus having the height commensurable with the length of the crystal grown. This is a particular problem in growing long crystals.
An improvement over the above apparatus is an apparatus for growing single crystals from a melt, comprising a frame-mounted and pressure-tight furnace housing horizontally split into two members, a crystal-pulling rod provided with a seed-crystal holder and vertically extending from external of the furnace housing into the top furnace member, a drive means for moving the crystal-pulling rod in both rotational and translational modes, and a crucible with a heater arranged in the furnace base member (see U.S. Pat. No. 3,865,554). The furnace base member is immovable on the frame while the top furnace member is swingable away together with the crystal-pulling rod and the seed-crystal holder.
The crystal grown is withdrawn from the furnace housing as follows. Following the step of pulling the crystal away from the melt the pressure in the growing chamber is equalized with atmosphere by letting the air in, if the crystal was grown in a vacuum, or by releasing the gas from the chamber, if the crystal was grown under an excessive pressure. Then the top furnace member is detached from the furnace base member and, together with the pulling rod and the seed crystal with the crystal grown suspended therefrom, is raised so that the lower end face of the top furnace member is above the upper end face of the crucible. Then the top furnace member together with the pulling rod and the crystal is transferred horizontally away from the furnace base member and the crystal is separated from the pulling rod after the latter was advanced from the top furnace member.
At first glance in the above apparatus the seed crystal and the crystal grown seem to be isolated from the outside cold air at least as long as they are inside the top furnace member since both the seed crystal and the crystal grown are in the heated air (or the gas wherein the crystal was grown) that fills the top furnace member.
However, the jointing plane of the furnace housing in the prior art apparatus is below the upper end face of the crucible, therefore when the top furnace member is raised, the outside cold air flows into the same member in amounts equal to the total volume of the crucible portion, of the heater and other of the structural members associated with the latter inside the furnace base member above the jointing plane which were inside the top furnace member prior to the raising thereof. Also as the top furnace member is horizontally transferred together with the crystal-pulling rod carrying the seed holder and the crystal grown, the outside cold air can flow also into the top furnace member due to turbulence produced by the transfer movement. Thus, the above described apparatus does not completely exclude contact of the seed crystal and the grown crystal from cold air, i.e. the thermal shock is possible.
And finally it is to be noted that in both the apparatus described above the single crystal suspended from the seed crystal may break away due to inertia as they are horizontally moved.